Image sensors on a thinned substrate have been designed to improve the calorimetric performance of the sensors while enabling the sensors to be illuminated via the backside of a very thin silicon layer. This arrangement prevents the dispersion of photons and electrons photogenerated in the substrate and therefore prevents optical crosstalk which would greatly impair the calorimetric performance since neighboring image pixels correspond to different colors.
The fabrication of an image sensor on a thinned substrate generally comprises the following steps, beginning with a normal silicon substrate a few hundred microns in thickness, allowing industrial-scale handling of wafers of about ten to twenty centimeters in diameter, this substrate being coated on the front side with an epitaxial layer of single-crystal silicon, possibly isolated from the rest of the substrate by an oxide layer in the case of SOI (silicon-on-insulator) substrates. The electronic circuitry necessary for the various functions of the sensor (essentially image acquisition) is produced on the front side of this single-crystal layer. Next, the substrate is bonded, via its front side that bears this circuitry, to a transfer substrate of sufficient thickness for industrial handling, and the initial silicon substrate is thinned down to a thickness of a few microns. The resulting very thin silicon thickness precludes industrial handling of the wafer, this being the reason for the presence of the bonded transfer substrate.
The signals coming from the electronic image acquisition component thus produced are in general exploited by other electronic devices that do not form part of the component.
To increase the complexity of the tasks accomplished by the electronic image acquisition component without increasing the size (and therefore the cost) of the thinned sensor itself, it would be desirable to combine an auxiliary electronic circuit with the thinned sensor in a way that minimizes the footprint and optimizes the industrial production process. One solution envisioned by the present invention is to bond two integrated circuits face to face, one circuit being the thinned image sensor and the other being an integrated electronic circuit electrically connected to the thinned sensor.
The problem of electrically connecting the two circuits is however not easy to solve. One solution that seems to be appropriate consists in using the flip-chip connection technique in which the sensor (before being thinned) has external metal connection pads, and the facing integrated circuit has metalizations exactly opposite them, indium balls being provided on the metalizations of one of the two circuits. By reflow of the indium balls, the sensor and the auxiliary integrated circuit are soldered together, pad to pad. This is necessarily done before the sensor is thinned, and the soldering must be carried out during the wafer-scale process on entire silicon wafers, that is to say before they are diced into individual integrated-circuit chips, both as regards the sensor and the associated integrated circuit.
Besides the fact that it is not easy to produce these pads of the thinned sensor, it is also necessary to provide pads for the connection to the outside of the assembly formed by the thinned sensor and its associated integrated circuit. This is because the pads of the sensor that served for soldering the two wafers face to face are no longer accessible.
Furthermore, when soldering in this manner, there are operations that can no longer be carried out on the backside of the thinned sensor, in particular if these operations require high treatment temperatures incompatible with the presence of indium solder joints.
Moreover, patent publication US 2006/0043438 teaches a process for combining a thinned sensor with a CMOS integrated circuit substrate. This process requires operations (notably to form openings in the silicon and to refill them with oxide) on the sensor before it is thinned, so as thereafter to be able to make a connection by copper plugs in the zones thus prepared.